///
    T[2] opIndex(size_t idx) pure
    {
        assert(idx < length);

        Coords coords = coordsInSquare(idx);

        import std.traits;
        static assert(isMutable!(typeof(coords))); // ok

        if(coords.x <= coords.y) // under diagonal line?
        {
            auto latestIdx = srcLength - 1;

            // Mirroring coords
            coords.x = latestIdx - coords.x; // Error: cannot modify const expression coords.x
            coords.y = latestIdx - coords.y - 1;
        }

        return [coords.x, coords.y];
    }